The production of the aforementioned multilayer structures generally requires the use of processes for bonding and transferring layer(s) between various wafers or substrates.
Among the various bonding processes, one of them, known as “molecular bonding adhesion,” consists of putting the surfaces to be bonded directly into intimate contact, without any additional material being placed between these surfaces. In such a case, it is said that the bonding takes place by “molecular adhesion” between the two surfaces.
This process makes it possible, in particular, to advantageously produce structures known under the acronym “SeOI” (which stands for “Semiconductor On Insulator”), in which an insulating layer, generally an oxide, is inserted between a thin film of semiconductor material and a receiver substrate or similar structures, known under the acronym “SOI,” in which the thin film is made of silicon.
It also makes it possible to produce structures known under the acronym “SOI UTBOX” (which stands for “Silicon On Insulator Ultra Thin Buried Oxide”), in which an oxide layer having a thickness of less than or equal to 50 nm (50 nanometers), or even less than or equal to 20 nm and greater than 1 nm is buried between a layer of silicon and a receiver substrate.
Such SeOI, SOI or SOI UTBOX structures may, for example, be manufactured by bonding a layer of semiconductor material (for example, silicon) from a donor substrate, onto a receiver substrate covered with an oxide layer and by transferring it to this oxide layer by then detaching the donor substrate. The structure obtained has a bonding interface between an oxide layer and a layer of semiconductor material.
However, in order to prevent the appearance of hydrogen blisters during the detachment annealing, especially in the case of SOI UTBOX, these structures are advantageously manufactured by bonding of a donor substrate covered with an oxide layer to a receiver substrate also covered with an oxide layer. Once bonded, the two oxide layers form only one layer.
However, after the finishing treatments carried out on the SOI UTBOX structure thus obtained, an incomplete stabilization (or strengthening) of the oxide/oxide bonding interface was observed, probably due to the trapping of water at this interface.
This incomplete stabilization is capable of interfering with the performances of the electronic devices or components that will be manufactured from these structures and that will include the ultra thin buried oxide layer.
However, for certain applications, the buried oxide (BOX) layer plays a significant electrical role (for example, in architectures of the “ground plane” or “back gate” type).
Any defect capable of impairing the electrical properties of the BOX, especially the charge density at the interface known to a person skilled in the art under the abbreviation “Dit,” and the charge of the oxide known under the abbreviation “Qbox,” or of compromising its homogeneity, may then prove very damaging.
In order to ensure satisfactory and reproducible electrical performances, it is necessary to complete the stabilization of the bonding interface, whether it is formed between two oxide layers or between one oxide layer and one silicon layer.
Stabilization is a microscopic phenomenon that reflects the establishment of atomic bonds (covalent bonds) between the two bonded layers, this being homogeneous over the whole of the bonding interface.
The treatments known from the prior art for SOI substrates consist of applying a stabilization annealing carried out at a temperature above 1100° C. for several hours.
Such an annealing lengthens and complicates the manufacturing process and increases its costs. Moreover, such an annealing is capable of degrading the quality of the thin film of the substrate. Indeed, beyond 1000° C., defects known as “slip lines” may be generated due to the appearance of localized stress zones at the points of contact between the substrate and the device intended to support it in the furnace.
Moreover, in some heterostructures comprising materials with different substrates having different thermal expansion coefficients (CTEs), such as silicon-on-sapphire (SOS), the molecular bonding adhesion between the oxide SOI layer and the sapphire layer is not strong enough to ensure good quality of the final layer transfer, especially during a thermal stabilization step. This step is necessary to ensure a bonding energy sufficiently strong over the entire surface of the plate in such a way in order to perform a grinding step. The thermal stabilization requires submission of the two bonded substrates to a rising temperature (100° C. to 180° C.). Because of their different CTE, the rising temperature causes a strong curvature of the bonding and a stress at the bonding interface, which is mainly focused on the edge of the bonded area for substrates having a circular shape. This accumulation of stress causes separations of the bonded substrates, quality degradations of the transfer and defects.
The objective of the invention is, therefore, to provide a process for stabilizing a bonding interface by molecular adhesion, which does not exhibit the aforementioned drawbacks of the prior art.
For this purpose, the invention relates to a process for stabilizing a bonding interface, located within a structure for applications in the fields of electronics, optics and/or optoelectronics, which comprises an oxide layer buried between an active layer and a receiver substrate, the bonding interface having been obtained by molecular adhesion.
In accordance with the invention, the process comprises irradiating this structure with a light energy flux provided by a laser, so that the flux, directed toward the structure, is absorbed by the energy conversion layer and converted to heat in this layer, and in that this heat diffuses into the structure toward the bonding interface, so as to thus stabilize the bonding interface.
According to the invention, the energy conversion layer can be formed on and/or in the active layer, but also can be the active layer.
According to other advantageous and non-limiting features of the invention, taken alone or in combination:                the fluence of the laser and the material constituting the energy conversion layer are chosen so as to bring the buried oxide layer to a temperature above 1200° C.;        the material constituting the energy conversion layer has a thermal conductivity of less than 20 W/m·K;        the bonding interface extends either between two oxide layers that, taken together, constitute the buried oxide layer, or between the buried oxide layer and the active layer, or between the buried oxide layer and the receiver substrate;        the irradiation of the structure is carried out by exposing the free surface of the receiver substrate, referred to as the “back face,” to the light energy flux, the material constituting this receiver substrate being transparent in the range of wavelengths of the light energy flux;        the irradiation can be located on a specific location, for example, the edges of the energy conversion layer;        the receiver substrate is made of silicon;        the receiver substrate is made from a material chosen from sapphire, aluminum oxide (Al2O3), aluminum nitride (AlN), silicon carbide (SiC) and quartz;        the laser is an infrared laser, the wavelength of which is greater than 9 μm;        the laser is a pulsed CO2 laser;        the buried oxide layer has a thickness of less than 50 nm, preferably between 1 nm and 50 nm;        the material constituting the active layer is a semiconductor material;        the semiconductor material is silicon;        the oxide constituting the buried oxide layer is chosen from silicon oxide (SiO2), aluminum oxide (Al2O3) and hafnium oxide (HfO2);        the energy conversion layer is made from a material chosen from silicon oxide (SiO2) and silicon nitride (Si3N4);        the energy conversion layer is made from silicon oxide (SiO2) and the stabilization treatment is followed by a step of removing this energy conversion layer.        